The present invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a patient""s body lumen, such as a blood vessel, to maintain the patency thereof. Stents are particularly useful in the treatment and repair of blood vessels after a stenosis has been compressed by percutaneous transluminal coronary angioplasty (PTCA), percutaneous transluminal angioplasty (PTA), or removed by atherectomy or other means, to help improve the outcome of the procedure and reduce the possibility of restenosis.
Stents are generally cylindrically shaped devices which function to hold open, and sometimes expand, a segment of a blood vessel or other arterial lumen, such as a coronary artery. Stents are usually delivered in a compressed condition to the target site and then deployed at that location into an expanded condition to support the vessel and help maintain it in an open position. They are particularly suitable for use to support and hold back a dissected arterial lining which can occlude the fluid passageway there through.
A variety of devices are known in the art for use as stents and have included coiled wires in a variety of patterns that are expanded after being placed intraluminally on a balloon catheter; helically wound coiled springs manufactured from an expandable heat sensitive metal; and self-expanding stents inserted in a compressed state for deployment into a body lumen.
One of the difficulties with prior art stents involves restenosis, which is recurrent stenosis, or narrowing, in a body lumen after a corrective procedure. Restenosis is a multifactorial problem believed to be caused by stimulatory processes stemming from the presence of thrombus, a foreign body reaction to the stent, and stent injury to the endothelium and lumen wall from deployment and the stent""s pressure against the lumen wall. A related difficulty for prior art stents involves use in connection with intercoronary brachytherapy. Brachytherapy is traditionally a procedure for treatment of cancer, or other proliferative diseases and the like, wherein radiation is used at or near the target site within the body. The radiation has an antiproliferative effect, meaning that rapidly dividing cells are affected most by the DNA damage induced by the radiation, which leads to apoptosis, or cell death, upon division. Thus, brachytherapy has the positive effect of potentially reducing restenosis after angioplasty or stenting, but it also presents new challenges for prior art stents.
Due to cellular damage and inhibition resulting from brachytherapy, a phenomenon known as positive vessel remodeling has been observed in vivo, which vessel lumen enlargement and, sometimes, medial thinning. Positive remodeling is beneficial to the extent that the vessel is still fully functional. The typical balloon expandable, prior art stent, because of its constant size after expansion, may either prevent positive remodeling from occurring or may be left exposed in the lumen after the lumen wall recedes. Such an exposed stent greatly increases the risk of thrombosis.
Due to the cellular damage and death resulting from brachytherapy, a phenomenon known as positive vessel remodeling has been observed in vivo, where new cells eventually grow and xe2x80x9cremodelxe2x80x9d the body lumen walls. The typical prior art stent, because of its constant radial expansion and force, may either prevent the positive remodeling from occurring or may be left exposed in the lumen after the lumen wall recedes.
Prior art stents typically fall into two general categories of construction. The first type of stent is expandable upon application of a controlled force, often through the inflation of the balloon portion of a dilatation catheter which, upon inflation of the balloon or other expansion means, expands the compressed stent to a larger diameter to be left in place within the artery at the target site. The second type of stent is a self-expanding stent formed from shape memory metals or super-elastic nickel-titanum (NiTi) alloys, which will automatically expand from a compressed state when the stent is advanced out of the distal end of the delivery catheter into the blood vessel. Stents manufactured from expandable heat sensitive materials usually allow for phase transformations of the material to occur, resulting in the expansion and contraction of the stent. Other self-expanding stents may use SIM alloys to allow the stent to move between contracted and expanded positions.
Typical stent delivery systems for implanting self-expanding stents at the target site include an inner lumen upon which the compressed or collapsed stent is mounted and an outer restraining sheath which is initially placed over the compressed stent prior to deployment. When the stent is to be deployed in the body vessel, the outer sheath is moved in relation to the inner lumen to xe2x80x9cuncoverxe2x80x9d the compressed stent, allowing the stent to move to its expanded condition.
Balloon expandable stents have the advantage of a high radial strength that is capable of holding open a tight stenosis. This stenosis is initially expanded by the force of a high pressure balloon. However, such a stent requires this dilitation to occur all at once. There is the alternative of simply deploying a self-expanding stent, and allowing the lumen to be slowly dilated over time to produce less injury. High levels of vessel injury are directly correlated with the greater levels of restenosis. However, a traditional manner of utilizing self-expanding stents requires predilating the lesion with a balloon, deploying the stent, and then postdilating the stent with another balloon. These many manipulations in the lesion increase the amount of vessel injury and endothelial denudation. Compared to this, the single step of deploying the self-expanding stent is attractive if the acute outcome is acceptable. The other issue, already mentioned, regarding balloon expandable stents is that if positive remodeling occurs, the stent is left exposed in the vessel. These two aspects, the ability to gradually expand the lesion over time, and expand with the vessel in a more passive sense, are compelling reasons to consider self-expanding stents over balloon expandable stents, particularly when used in conjunction with brachytherapy. Use of self-expanding stents to either hold open a previously tight stenosis, or to start the process of opening a lesion without completing predilatation, can require a high initial radial force. Compared to balloon expandable stents, usually of 316L stainless steel, achieving a high initial radial strength with NiTi alloys can be more of a challenge.
Currently, a high initial radial force for a self-expanding stent is accomplished by either oversizing the stent (to increase the strength) or by placing more metal in the stent (i.e., by making the struts thicker and wider). However, oversizing may lead to excessive vessel injury as the stent continues to exert a force against the vessel wall once implanted. For this reason, there is currently a concern that oversizing the stent by a large amount, in order to exert a large initial force, may be detrimental, as the stent will keep pushing the vessel wall possibly causing injury. Moreover, oversizing or increasing the amount of metal in the stent changes the way in which the stent functions biologically. For example, increasing the amount of metal results in a stent having thicker struts and increases the stent to artery ratio, which may not be desirable for every stent application. The stent to artery ratio is defined as the percentage of the vessel wall area covered by the stent struts. With balloon expandable stents, there is a known and preferred range of stent to artery ratio, strut thickness and width. When engineering a self-expanding stent, one challenge is designing a stent which expands to a final diameter, as dictated by the reference vessel size of the body lumen in which the stent is to be implanted, while exerting a radial force at this final diameter that is not injurious to the body lumen wall.
What has been needed and heretofore unavailable is a self-expanding stent that has a time variable radial force. In one configuration, when first deployed, such a stent should exert the necessary force to expand against the lesion, dissected arterial lining, thrombus, embolus, or other lumen abnormality and maintain the patency of the lumen. This process can occur actively, or over a short interval of time. Later, when the vessel wall is remodeling, or simply because the vessel has reached its optimum size, the stent should exert a relatively lower force on the wall, thereby minimizing the risk of injury and allowing more complete repair and healing of the lumen. In a second configuration, the stent should exert a lower radial force when first deployed, this force increasing at a later time. This will facilitate deployment of a self-expanding stent as the stent expansion forces increase the friction against a restraining sleeve or sheath. This configuration is also beneficial when the body lumen is in need of only minor repair and aggressive expansion is not initially desired. The present inventions disclosed herein satisfy these and other needs.
The present invention is directed to a self-expanding stent having a configuration which allows the radial force exerted by the stent to vary with time. The stent is comprised of a network of resilient struts configured with undulations defining flexible segments which are interconnected to cooperatively form the flexible stent. The flexible segments are bendable to allow the stent to move between its collapsed and expanded positions. Biodegradable material is strategically incorporated into the strut pattern (via the flexible segments) to either encourage or restrict bending or flexing of the flexible segments toward their preferred positions. As the biodegradable material degrades over time, its effect of producing either an increase or decrease of the rate of radial expansion of the stent is gradually lessened.
In one aspect of the present invention, the stent is comprised of struts that have flexible segments which include peaks and valleys that aid in the even distribution of radial expansion forces. The various flexible segments can have, inter alia, U, V, Y and W shapes which bend and flex to allow the composite stent device to expand from its collapsed position. Each of these particular flexible segments includes at least one strut junction where the struts forming the flexible segment meet. The bending or flexing of the strut junctions provides the outward radial force which is developed by the stent to move between its collapsed position to the expanded position. Biodegradable material, which acts as a bend control member, can be strategically placed, for example, within the strut junction of the flexible segment to directly influence the amount of radial force that will be exerted by that particular flexible segment. The amount of radial force exerted by the flexible segment will change once the biodegradable material begins to erode. For example, biodegradable material can be placed within select strut junctions when the stent is in its fully opened, relaxed shape or, alternately, when it is expanded beyond its fully opened and relaxed shape. When the stent is crimped down to its collapsed position, the strut junctions will be placed under greater stress with the biodegradable material in place than they would without the presence of the material. When implanted, the stent exerts a high initial radial strength since the biodegradable material has been placed in compression within the strut junctions and is exerting additional outward force on the struts to increase the overall outward radial force of the composite stent. This radial force drops somewhat as the stent expands further, and then, after the stent is placed in the body vessel, the biodegradable material will gradually degrade. This erosion of the biodegradable material gradually reduces the resistance of the flexible segment to bending and, correspondingly, the amount of radial force exerted by the stent against the lumen wall.
In another aspect of the present invention, the biodegradable material is selectively incorporated into the stent pattern under tension. In this aspect, the radial force generated by the deployed stent would be initially lower than it normally would be without the presence of the biodegradable material and would increase to its full radial strength after the material begins to biodegrade. Depending upon the structure of the struts, the biodegradable material can be placed directly on existing struts at strut junctions or on specially created structures formed on the flexible segments which are adapted to receive the biodegradable material. A number of different structures can be incorporated into a strut pattern to create these regions where the biodegradable material can be applied to help control the amount of bend exerted at the strut junctions to obtain the desired radial force over time characteristics for that particular stent.
In another aspect of the invention, the straight portions of the struts can be formed with decreased cross-sectional areas (strut width) at certain locations which will result in these struts producing less radial force due to the added flexing of the struts at these locations. For example, a slot can be cut into the straight struts and filled with biodegradable material to produce an initial high radial force when the stent is initially implanted in the patient""s vasculature. In this manner, the biodegradable material acts to initially stiffen the strut against bending in the region where the strut has a reduced cross-sectional area. Since these regions are under a sheer moment, as the biodegradable material erodes, the struts will have greater flexibility due to the reduced cross-section of the strut, thus lowering the composite radial expansion force exerted by the stent. Using this technique, a large initial radial force can be imparted by the stent which will gradually decrease as the biodegradable material erodes within the patient""s vasculature since certain struts become narrower and flex to a greater extent.
In still another aspect of the present invention, one or more filaments of biodegradable material that forms a hoop can be utilized to encircle the entire stent. The filament(s) would expand with the stent and would be left in place within the patient""s vasculature. The filament(s) would be placed on the stent in tension thus initially reducing the amount of radial expansion force exerted by the stent. As the filament(s) begins to degrade, the tension placed on the stent decreases which, in turn, allows the stent to deploy to its fully expanded position, and increases the radial force imparted by the stent against the body vessel.
The present invention is also directed to a method of making a self-expanding stent with integrated biodegradable material. The method includes forming the stent pattern and selectively placing the biodegradable material into the strut junctions or reduced strut regions using a solution coating technique. Alternatively, the biodegradable material can be deposited using a manual or automated process.
The present invention is intended for use with any self-expanding stent. This includes stents for the coronary, carotid, neurological, renal, hepatic, iliac, biliary, popliteal, prostrate, femoral or other peripheral vasculature. The stent itself can be made from any of the materials that are used to manufacture self-expanding stents, including, but not limited to, nitinol, elgiloy, and other shape-memory metals. There are no limitations on the stent length or diameter, strut thickness, strut width, or strut pattern of the stent. Variable amounts of biodegradable materials may be used and there are no minimal or maximum amounts that need to be incorporated into the stent. Additionally, more than one type of biodegradable material can be incorporated into the stent.
The stent of the present invention is particularly useful for implantation in body lumens known to have stenotic lesions, dissected arterial linings, thrombus, embolic deposits or other abnormalities because the higher initial radial expansion force can move the tissue and maintain the patency of the lumen. Then, when the biodegradable material has become resorbed, the decreased radial expansion force of the stent, again achieved due to the degradation of the biodegradable material incorporated in the stent structure, helps enable the lumen wall to remodel, or repair itself, to remain healthy, and to hopefully avoid restenosis.